Ab Initio Molecular Dynamics Simulation of Water Transport through Short Carbon Nanotubes

Abstract

Water transport through short single-walled (6, 6) carbon nanotubes (CNTs) was investigated with ab initio molecular dynamics (AIMD) simulation at different temperatures. The water molecules under extreme confinement present a one-dimensional jagged pattern owing to hydrogen bonding, with the near-perfect alignment of the dipole orientations. CNTs ending with dangling bonds can promote water dissociation near the entrance and the occurrence of dipole flipping along the water wire at high temperatures, accompanied by the formation of D defects and L defects in the hydrogen-bond network. In contrast, dissociation of water molecules rarely takes place if the dangling bonds at the ends of the CNTs are terminated with H atoms. Angular jumps of water molecules are commonplace inside the narrow CNTs, implying a low-energy barrier for hydrogen-bond exchange among water molecules in narrow CNTs. The simulation results demonstrate the high activity of dangling bonds at the ends of short CNTs, accompanying passivation processes and their profound impact on water structure and transport, which is important for diverse technological applications.

Figure 1

Narrow CNTs in liquid water with dangling bonds at the ends, Case I (a) or with the ends capped by hydrogen atoms, Case II (b). Fluctuation of the CNT structure is measured in terms of the root-mean-square deviation (RMSD) from the initial configuration of CNTs in Case I (c) and Case II (d) at 300, 400, and 500 K.

Figure 2

(a) Four types of hydrogen bonds between water molecules in narrow CNTs. (b) Water molecules inside the CNTs form a jagged pattern.

Figure 3

Probability density of the angles between the O_iO_i+1 vectors and the axial direction is predicted by the AIMD simulation for Case I (a) and Case II (b) at 300, 400, and 500 K.

Figure 4

Projections of O and H atoms on the cross-sectional area of the CNTs in Case I (a) and Case II (b). The dots in the figures are projections of the atomic positions (red for oxygen atoms and gray for hydrogen atoms). The radii marked in the figures are the 99th percentile of all atomic positions (blue for oxygen atoms and green for hydrogen atoms).

Figure 5

Statistical histograms for the relative dipole moments of water molecules in the CNTs with dangling bonds at the ends (a) and without dangling bonds (b). The axial (orange) and radial (green) components of the dipole reflect the orientations of confined water molecules.

Figure 6

(a) Z-axis components of relative dipole moments of water molecules confined to the CNTs changes with simulation at 400 K in Case I. Indices 0, 1, 2, and 3 correspond to the water molecules distributed along the z-axis direction. (b) D defect at 400 K in Case I. (c) L defect at 500 K in Case I.

Figure 7

Analysis results of hydrogen bonds formed inside the CNTs of Case I (a) and Case II (b) at different temperatures (blue for 300 K, orange for 400 K, and green for 500 K). From left to right, the distributions of the number of hydrogen bonds, hydrogen bond types, hydrogen bond angles, and hydrogen bond lengths are plotted in this order.

Figure 8

Hydrogen bond angular jump.

Figure 9

(a) Variation of the bond angle formed by the O atom (vertex) and H atom of one water molecule with the O atom of the neighboring water molecule inside the CNTs of Case I with the simulation time at 300 K. (b) Statistical distribution of the above angles.

Figure 10

(a) Three scenarios for the variation of the O–H distance with simulation time. (b) Proton conduction among some water molecules aligns the hydrogen-bonding network. (c) Variation of the O–H distance of water molecules involved in the proton conduction with the simulation time.

Figure 11

Dangling bonds capped by hydrogen and hydroxyl at 300 K (a), by the ether bond at 400 K (b), and by carbonyl at 500 K (c). (d) Radial distribution functions (RDF) of C atoms carrying dangling bonds and O atoms in Case I. The RDF of the C atom with the same locations as in Case I and the O atom in Case II.